The invention relates to a method for assisting an operator of a vehicle in adjusting a nominal steering angle at steerable wheels of the vehicle for the vehicle stabilization, wherein an additional steering torque is applied to the steering line of the vehicle, which is determined dependent on a difference between a nominal steering angle and an instantaneous steering angle.
Further, the invention relates to a device for assisting an operator of a vehicle in adjusting a nominal steering angle on at least one steerable wheel of the vehicle, comprising a means for adjusting an additional steering torque, which is determined by a control unit dependent on a difference between the nominal steering angle and an instantaneous steering angle.
When braking on an inhomogeneous roadway with different coefficients of friction on the left and the right longitudinal side of the vehicle, asymmetric brake forces may develop which cause a yaw torque that will induce rotational movement of the vehicle about its vertical axis. To prevent the vehicle from skidding, the driver is required to build up a compensating yaw torque by way of appropriate steering movements, said yaw torque counteracting the torque caused by the asymmetric brake forces. As this occurs, locking of the wheels should be avoided also on the vehicle side with the low coefficient of friction because the major reduction of the transmittable cornering force of a wheel, which comes along with wheel lock, can prevent the development of the necessary compensation torque. The situation at topic, hence, places high demands which regularly asks too much of inexperienced drivers in particular.
In vehicles with an anti-lock system (ABS), a controller prevents the wheels from locking. In situations of the type mentioned, a control strategy that is generally used has the objective of effectively decelerating the vehicle by way of a maximum high brake pressure in the wheel brakes on the vehicle side exhibiting the higher coefficient of friction, on the one hand. On the other hand, the driver must not be overloaded by a yaw torque caused due to different brake forces on the side with high and low coefficients of friction. Therefore, in the situations at topic, the control is carried out in such a manner that a difference between the brake pressures on the side with high and low coefficients of friction at the front axle of the vehicle develops only slowly in order to impart sufficient time to the driver to perform stabilizing steering movements. In addition, the brake pressure at both wheel brakes of the rear axle is limited to the value admitted for the low coefficient-of-friction side (‘select low’) to allow a sufficient rate of cornering force to build up at the rear axle, with a view to stabilizing the vehicle by steering interventions.
The presented measures enable the driver to master the vehicle easily, however, the coefficient-of-friction potential of the high coefficient-of friction side is not optimally utilized to decelerate the vehicle. Therefore, it has been proposed to include the steering angle of the steerable wheels of the vehicle into driving stability control and to have a controller and an appropriate actuator actuated by the latter adjust a steering angle, which brings about the necessary compensation yaw torque. The vehicle may thus be stabilized more quickly and more reliably so that a more ‘aggressive’ ABS control strategy can be chosen, which permits a more effective brake intervention and consequently a quicker deceleration of the vehicle. Also, it is avoided that the vehicle will be skidding due to insufficient or misdirected steering interventions of the driver.
International patent application WO 02/074638 A1 discloses applying an additional steering torque to the steering line of the vehicle, which is established from a deviation between the steering angle that is adjusted by the driver and a steering angle requirement, and which is set by an electronic power steering system. The steering angle requirement is determined from the estimated yaw torque that acts on the vehicle, the yaw rate of the vehicle, and the lateral acceleration of the vehicle.
The result is that the steering angle that is determined by the control unit is not adjusted directly, but the additional steering torque assists the driver when adjusting the steering angle.
However, the problem encountered in a control of this type involves that the additional steering torque will adopt excessively high values in a braking operation on a roadway with low coefficients of friction when the control is tuned to high coefficients of friction. When the control is tuned to low coefficients of friction, the additional steering torque is too low when braking on a roadway with high coefficients of friction in order to assist the driver effectively. This may cause erroneous or inappropriate steering interventions of the driver, which is frequently noticed as being uncomfortable.
In view of the above, an object of the invention involves improving a method of the same type to such extent that the vehicle can be stabilized more reliably and more comfortably in a large number of possible driving situations.